1. Field of the Invention
The present invention relates to control circuit improvements for tensile test controllers. At the present time, servo controlled testing systems are well known, and are used for various types of tests. These include servo controlled actuators that perform fatigue tests, and also servo controlled actuators for tensile testing of specimens to determine material characteristics.
2. Description of Prior Art
Servo controlled test systems operate by minimizing an electronic error signal, so that a programmed loading can be achieved. In elementary form an electrical command input signal is provided, generally in the form of one or more monotonic ramp signal(s) having a desired voltage level relative to time, and then providing a feedback signal(s) from the load frame being operated, which is compared with the input signal at an input summing junction so that any error between the desired command signal and the feedback signal will result in a change in the drive to the servovalve and thus a change in the actuator operation.
The servoloops are commonly operated in "load control", that is, the command signal indicates the load which is desired to be exerted by the actuator and the feedback signal from a load cell or other force sensing transducer indicates the actual load exerted. Another common mode of servoloop operation is "stroke control" in which the input command signal represents desired displacement of the actuator, and the feedback would be from a stroke or displacement transducer such as an LVDT. When testing specimens in tensile tests, it is also common to have an extensometer mounted on the specimen which measures the strain on the specimen, and in such instances it is normal to alternatively utilize the strain feedback signal from the extensometer to control the actuator as a function of a programmed or commanded strain. In other words, a command input signal will indicate the amount of desired strain in the specimen across a period of time, and the strain feedback signal is then used for controlling the actuator through servo controls.
When testing a specimen in tension, it sometimes happens that the extensometer will slip, or become loose, or a wire could break, and if that happens the testing system essentially is uncontrolled if the system happens to be operating in strain control.
When the system is operated initially in load control on a test specimen, as the specimen starts to yield, the specimen reacted loads can drop off substantially, causing undesired accelerated straining of the specimen. Depending on the specimen characteristics, there may be undesired strain acceleration during specimen post yield when ramping initially in a straight load control.
The present invention solves some of these problems by providing an automatic control wherein load and displacement are used in sequence, and also in one form of the invention, cascade strain control, for preventing unwanted excursions of the actuator in case the strain feedback signal is lost. The use of load and displacement control in sequence permits achieving accurate control of the test even during specimen yielding, and when the elastic and plastic portion of the test is known to be complete, final accelerated fracture of the specimen can occur.
Various circuits have been advanced for driving (controlling) servo valves, and various linearization schemes also have been utilized. Bumpless control transfer circuits are used, for changing from one control mode to another, and all of these are operated using essentially the same types of controls as that shown herein with the exception that the present device has the cascade control for fail-safe operations in extensometer control testing.